JP7456223B2 - Actinic radiation curable ink and image forming method - Google Patents

Description

The present invention relates to an actinic radiation-curable ink and an image forming method.

Inkjet recording methods are used in various printing fields because images can be formed easily and inexpensively. 2. Description of the Related Art Actinic radiation curable inks containing an actinic radiation polymerizable compound as a liquid component are known as inks used in inkjet recording systems. When actinic radiation is irradiated, actinic radiation-curable ink is cured by polymerization of an actinic radiation-polymerizable compound, thereby firmly adhering the coloring material to the recording medium. BACKGROUND ART Image forming methods using actinic radiation-curable inks have been attracting attention in recent years because they can form images with high scratch resistance and adhesion even on recording media that do not have ink absorption properties.

As a type of actinic radiation curable ink, ink that undergoes a phase change depending on temperature due to the addition of a gelling agent is known.

For example, Patent Document 1 discloses an actinic radiation-curable ink for inkjet that includes a white colorant, a curable monomer, and a gelling agent, and whose phase changes with temperature changes. According to Patent Document 1, the above-mentioned ink melts and has a low viscosity at the temperature when it is ejected, but on the other hand, when the ink lands on a base material such as a recording medium or an intermediate transfer body and the temperature changes, the viscosity decreases. It is said that the particles rapidly increase in size and solidify to form a predetermined pattern. In the ink described in Patent Document 1, a white pigment such as titanium oxide is used as the white colorant.

Japanese Patent Application Publication No. 2007-63553

As described in Patent Document 1, a white actinic radiation curable ink containing a gelling agent is known.

However, according to the findings of the present inventors, the white actinic radiation-curable ink containing a gelling agent causes cracks in the formed image. The above-mentioned cracks are particularly noticeable when an image is formed using an actinic radiation-curable ink containing a white pigment.

The present invention has been made in view of the above circumstances, and provides an actinic radiation-curable ink that contains a white pigment and is less likely to cause cracks in the formed image, and an actinic radiation-curable ink that contains such an actinic radiation-curable ink. An object of the present invention is to provide an image forming method for use in the present invention.

An actinic radiation-curable ink according to an embodiment of the present invention for solving the above problems is an actinic radiation-curable ink containing a hydrophobized white pigment, an actinic radiation polymerizable compound, and at least two types of wax. It's ink. At least one of the at least two types of waxes is a wax having a branched structure.

Further, an image forming method according to an embodiment of the present invention for solving the above problems includes a step of applying the actinic radiation-curable ink to a recording medium, and applying actinic radiation to the applied actinic radiation-curable ink. and a step of irradiating.

The present invention provides an actinic radiation curable ink that contains a white pigment and is less likely to cause cracks in the formed image, and an image forming method using such an actinic radiation curable ink.

FIG. 1 is a side view showing the concept of an inkjet image forming apparatus according to an embodiment of the present invention.

Embodiments of the present invention will be described in detail below with reference to the drawings. Note that the present invention is not limited to the following embodiments.

[Active radiation curable ink]
The actinic radiation curable ink (hereinafter also simply referred to as "white ink") according to an embodiment of the present invention comprises a hydrophobically treated white pigment, an actinic radiation polymerizable compound, and at least two types of wax. include. In the actinic radiation curable ink, at least one of the at least two types of waxes is a wax having a branched structure.

According to the findings of the present inventors, inks for forming white images usually use hydrophilic compounds such as titanium oxide as white pigments. These white pigments have low affinity with waxes that are hydrophobic, and voids tend to form between the pigment and the wax. These voids can become the starting points for cracks, or the cracks can connect to each other and grow through these voids, so it is believed that inks containing white pigments are prone to cracking.

Therefore, it is thought that if the white pigment is subjected to a hydrophobic treatment to make it difficult to form voids between the white pigment and the wax, cracks will be less likely to occur.

Further, as a result of further intensive research by the present inventors, it was discovered that the above-mentioned cracks occur immediately after the white ink is applied to the recording medium and before the ink is cured by irradiation with actinic radiation. From this fact, it is considered that the cracks are caused by volumetric shrinkage due to crystallization of wax in the ink applied to the recording medium.

Since wax crystals are relatively large in size, they are more likely to be oriented in the plane direction (parallel to the recording medium) than in the thickness direction of the ink in the ink applied to the recording medium. It is thought that volumetric contraction occurs due to crystallization while being oriented in the plane direction, making it easier for cracks that extend in the plane direction to occur.

In order to suppress the occurrence of cracks due to this orientation in the plane direction, the white ink according to this embodiment contains at least two types of waxes having mutually different structures. These waxes having mutually different structures mix with each other in the white ink applied to the recording medium and crystallize at the same time. As a result, the white ink can have a smaller wax crystal size and a more random orientation of the crystals than an ink containing a single wax. It is thought that this suppresses the orientation in the above-mentioned plane direction and suppresses the occurrence of cracks.

Further, the white ink contains a wax having a branched structure as at least one of the two types of waxes. Wax having a branched structure has carbon atom chains extending in a plurality of different directions, so that the carbon atom chains tend to be arranged in a plurality of directions. It is thought that this also makes it possible for the white ink to make the crystal size of the wax smaller and to make the orientation of the crystals more random, suppressing the orientation in the above-mentioned plane direction and suppressing the occurrence of cracks.

For these reasons, it is thought that the white ink is less likely to cause cracks in the formed image, even though it contains a white pigment.

(White pigment)
The white pigment may be any pigment that imparts a white color to the cured film formed by curing the white ink. Examples of the white pigment include inorganic pigments such as titanium oxide, zinc oxide, calcium carbonate, barium sulfate, and aluminum hydroxide. Among these, titanium oxide is preferred.

The crystal form of the titanium oxide may be any of the rutile type, anatase type, and brookite type, but from the viewpoint of making it easier to reduce the particle size of the white pigment, the anatase type, which has a small specific gravity, is preferable, and the formed image From the viewpoint of further enhancing the hiding property, a rutile type material having a high refractive index in the visible light region is preferable.

The white pigment is surface-treated with an organic substance to make it hydrophobic. The white pigment fine particles have a higher degree of surface hydrophilicity than fine particles of other pigments (organic pigments, etc.) used in actinic radiation curable inks. Therefore, compared to fine particles of other pigments, the fine particles of the white pigment are less likely to attract hydrophobic wax close to the fine particles in the ink, and are more likely to generate the above-mentioned voids. In order to suppress the occurrence and growth of cracks due to the voids, the white pigment is subjected to a hydrophobic treatment.

Further, when the white pigment is surface-treated with an organic substance, the dispersant is easily adsorbed to the pigment surface and aggregation of the pigments is suppressed, so that the storage stability of the white ink can be further improved.

The method of the above-mentioned hydrophobization treatment is not particularly limited. For example, the white pigment is preferably surface-treated with a polyol, a siloxane compound, or a silane coupling agent, and more preferably with a siloxane compound.

Examples of the above-mentioned polyols include polyols having 4 or more and 10 or less carbon atoms. Examples of the polyol having 4 to 10 carbon atoms include ethylene glycol, 2-methyl-1,2,3-propanetriol, pentaerythritol, trimethylolethane, and trimethylolpropane.

Examples of the siloxane compounds include various silicone oils including methylhydrogenpolysiloxane, dimethylpolysiloxane, methylphenylpolysiloxane, and the like.

Examples of the silane coupling agents include methyltrimethoxysilane, ethyltrimethoxysilane, hexyltrimethoxysilane, octyltrimethoxysilane, decyltrimethoxysilane, octadecyltrimethoxysilane, dimethyldimethoxysilane, octyltriethoxysilane, and Various alkylsilanes including n-octadecyldimethyl(3-(trimethoxysilyl)propyl)ammonium chloride, etc., various fluoroalkylsilanes including trifluoromethylethyltrimethoxysilane, and heptadecafluorodecyltrimethoxysilane, vinyltrimethoxy silane, and γ-aminopropyltrimethoxysilane.

Note that the surface treatment agent may be fatty acids including isostearic acid and stearic acid, metal salts thereof, known surfactants, and the like.

The surface treatment of the white pigment can be performed by a known method. For example, titanium oxide particles, a surface treatment agent, and a solvent may be mixed in a stirring mill, and then the solvent may be removed. Before the above mixing, the titanium oxide particles, the surface treatment agent, and the solvent may be premixed to form a slurry.

The hydrophobized white pigment preferably has a carbon atom content of 0.25% by mass or more and 2.5% by mass or less, and 0.30% by mass or more and 1.5% by mass based on the total mass. % or less, and even more preferably 0.35% by mass or more and 1.0% by mass or less. When the content of carbon atoms is 0.25% by mass or more, it is possible to make it more difficult to form voids between the white pigment and the wax, thereby making it more difficult to cause cracks. In addition, when the content of carbon atoms is 2.5% by mass or less, the wax is not drawn too close to the hydrophobized titanium oxide, and the amount of wax deposited on the surface of the cured film is maintained, thereby increasing the hardness of the cured film. can be sufficiently increased.

The content of carbon atoms can be a value measured by a combustion method.

The hydrophobized white pigment preferably has an average particle size of 50 nm or more and 500 nm or less, and more preferably 100 nm or more and 300 nm or less. When the average particle size is 50 nm or more, an image with sufficient concealment can be formed. On the other hand, when the average particle size is 500 nm or less, the hydrophobized titanium oxide can be stably dispersed, and the storage stability and ejection stability of the white ink can be improved.

The average particle diameter can be a value measured by a dynamic light scattering method.

The content of the hydrophobized white pigment is preferably 5% by mass or more and 30% by mass or less, more preferably 8% by mass or more and 20% by mass or less, based on the total mass of the white ink. preferable. When the content of the white pigment is 5% by mass or more, the effect of suppressing cracks by suppressing the generation of voids between the white pigment and the wax can be more effectively exerted. When the content of the white pigment is 30% by mass or less, the amount of wax near the surface of the cured film can be controlled more appropriately, and the hardness of the cured film can be sufficiently increased.

(Active radiation polymerizable compound)
The actinic radiation polymerizable compound is a compound that polymerizes and crosslinks upon irradiation with actinic radiation. Examples of actinic radiation polymerizable compounds include radically polymerizable compounds and cationic polymerizable compounds. Among these, the actinic radiation polymerizable compound is preferably a radical polymerizable compound.

Note that examples of the active rays include electron beams, ultraviolet rays, α rays, γ rays, and X rays. Among these, ultraviolet rays and electron beams are preferred, and ultraviolet rays are more preferred.

The radical polymerizable compound is a compound (monomer, oligomer, polymer, or mixture thereof) having an ethylenically unsaturated bond capable of radical polymerization. The radical polymerizable compound may be used alone or in combination of two or more kinds.

Examples of compounds having radically polymerizable ethylenically unsaturated bonds include unsaturated carboxylic acids and their salts, unsaturated carboxylic acid ester compounds, unsaturated carboxylic acid urethane compounds, unsaturated carboxylic acid amide compounds and their anhydrides, Examples include acrylonitrile, styrene, unsaturated polyester, unsaturated polyether, unsaturated polyamide, and unsaturated urethane. Examples of unsaturated carboxylic acids include (meth)acrylic acid, itaconic acid, crotonic acid, isocrotonic acid, maleic acid, and the like.

The radically polymerizable compound is preferably an unsaturated carboxylic acid ester compound, and more preferably a (meth)acrylate. In addition, in this specification, "(meth)acrylate" means acrylate or methacrylate, and "(meth)acrylic" means acrylic or methacrylic.

Examples of monofunctional (meth)acrylates include isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, isomylstyl (meth)acrylate, isostearyl (meth)acrylate, 2-ethylhexyl-diglycol (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 2-(meth)acryloyloxyethyl hexahydrophthalic acid, butoxyethyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate ) acrylate, methoxydiethylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, methoxypropylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, 2-hydroxy Ethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 2-(meth)acryloyloxyethylsuccinic acid, 2-(meth)acryloyloxyethyl phthalate The acids include 2-(meth)acryloyloxyethyl-2-hydroxyethyl-phthalic acid and t-butylcyclohexyl (meth)acrylate.

Examples of polyfunctional (meth)acrylates include triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, and polypropylene glycol di(meth)acrylate. (meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, Dimethylol-tricyclodecane di(meth)acrylate, PO adduct di(meth)acrylate of bisphenol A, neopentyl hydroxypivalate glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, polyethylene glycol diacrylate and Bifunctional (meth)acrylates including tripropylene glycol diacrylate, as well as trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate , ditrimethylolpropane tetra(meth)acrylate, glycerin propoxytri(meth)acrylate, and pentaerythritol ethoxytetra(meth)acrylate.

The radically polymerizable compound preferably contains (meth)acrylate modified with ethylene oxide or propylene oxide (hereinafter also simply referred to as "modified (meth)acrylate"). Modified (meth)acrylates have higher photosensitivity. Furthermore, modified (meth)acrylate is more easily compatible with other components even at high temperatures. Furthermore, since modified (meth)acrylates have less curing shrinkage, they are less likely to cause printed matter to curl when irradiated with actinic radiation.

Examples of cationically polymerizable compounds include epoxy compounds, vinyl ether compounds, oxetane compounds, and the like.

Examples of the above epoxy compounds include 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate, bis(3,4-epoxycyclohexylmethyl)adipate, vinylcyclohexene monoepoxide, ε-caprolactone-modified 3, 4-Epoxycyclohexylmethyl 3',4'-epoxycyclohexanecarboxylate, 1-methyl-4-(2-methyloxiranyl)-7-oxabicyclo[4,1,0]heptane, 2-(3,4 - cycloaliphatic epoxy resins such as epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexanone-meta-dioxane and bis(2,3-epoxycyclopentyl) ether, diglycidyl ether of 1,4-butanediol , diglycidyl ether of 1,6-hexanediol, triglycidyl ether of glycerin, triglycidyl ether of trimethylolpropane, diglycidyl ether of polyethylene glycol, diglycidyl ether of propylene glycol, ethylene glycol, propylene glycol, and glycerin. Aliphatic epoxy compounds including polyglycidyl ethers of polyether polyols obtained by adding one or more alkylene oxides (such as ethylene oxide and propylene oxide) to aliphatic polyhydric alcohols, and bisphenol A or Di- or polyglycidyl ethers of alkylene oxide adducts thereof, di- or polyglycidyl ethers of hydrogenated bisphenol A or its alkylene oxide adducts, and aromatic epoxy compounds including novolak-type epoxy resins.

Examples of the vinyl ether compounds include monovinyl ether compounds including ethyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, octadecyl vinyl ether, cyclohexyl vinyl ether, hydroxybutyl vinyl ether, 2-ethylhexyl vinyl ether, cyclohexanedimethanol monovinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, isopropenyl ether-o-propylene carbonate, dodecyl vinyl ether, diethylene glycol monovinyl ether, and octadecyl vinyl ether, as well as di- or trivinyl ether compounds including ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, propylene glycol divinyl ether, dipropylene glycol divinyl ether, butanediol divinyl ether, hexanediol divinyl ether, cyclohexanedimethanol divinyl ether, and trimethylolpropane trivinyl ether.

Examples of the oxetane compounds include 3-hydroxymethyl-3-methyloxetane, 3-hydroxymethyl-3-ethyloxetane, 3-hydroxymethyl-3-propyloxetane, 3-hydroxymethyl-3-n-butyloxetane, -Hydroxymethyl-3-phenyloxetane, 3-hydroxymethyl-3-benzyloxetane, 3-hydroxyethyl-3-methyloxetane, 3-hydroxyethyl-3-ethyloxetane, 3-hydroxyethyl-3-propyloxetane, 3 -Hydroxyethyl-3-phenyloxetane, 3-hydroxypropyl-3-methyloxetane, 3-hydroxypropyl-3-ethyloxetane, 3-hydroxypropyl-3-propyloxetane, 3-hydroxypropyl-3-phenyloxetane, 3 -Hydroxybutyl-3-methyloxetane, 1,4bis{[(3-ethyl-3-oxetanyl)methoxy]methyl}benzene, 3-ethyl-3-(2-ethylhexyloxymethyl)oxetane and di[1- Includes ethyl(3-oxetanyl)]methyl ether and the like.

The content of the active radiation polymerizable compound is preferably 1% by mass or more and 97% by mass or less, more preferably 10% by mass or more and 95% by mass or less, and even more preferably 30% by mass or more and 95% by mass or less, based on the total mass of the white ink.

(Active ray polymerization initiator)
The actinic radiation curable ink may contain an actinic radiation polymerization initiator.

The actinic radiation polymerization initiator is a compound that initiates polymerization and crosslinking of the actinic radiation polymerizable compound upon irradiation with actinic radiation. In addition, when polymerization and crosslinking of the actinic radiation polymerizable compound can be initiated without an actinic radiation polymerization initiator, such as when forming an image by irradiation with an electron beam, the white ink does not contain an actinic radiation polymerization initiator. It's okay.

The actinic radiation polymerization initiator can be a radical initiator when the white ink has a radically polymerizable compound, and can be a cationic initiator (photoacid generator) when the white ink has a cationically polymerizable compound. I can do it.

Radical polymerization initiators include intramolecular bond cleavage type radical polymerization initiators and intramolecular hydrogen abstraction type radical polymerization initiators.

Examples of intramolecular bond cleavage type radical polymerization initiators include diethoxyacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, benzyl dimethyl ketal, 1-(4-isopropylphenyl)-2 -Hydroxy-2-methylpropan-1-one, 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone, 1-hydroxycyclohexyl-phenylketone, 2-methyl-2-morpholino(4 -Methylthiophenyl)propan-1-one, acetophenone-based initiators including 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone, benzoin, benzoin methyl ether, benzoin isopropyl ether, etc. acylphosphine oxide initiators including 2,4,6-trimethylbenzoindiphenylphosphine oxide, and benzyl and methylphenylglyoxy esters.

Examples of intramolecular hydrogen abstraction type radical polymerization initiators include benzophenone, methyl o-benzoylbenzoate, 4-phenylbenzophenone, 4,4'-dichlorobenzophenone, hydroxybenzophenone, 4-benzoyl-4'-methyl-diphenyl. Benzophenone-based initiators, including sulfide, acrylated benzophenone, 3,3',4,4'-tetra(t-butylperoxycarbonyl)benzophenone, and 3,3'-dimethyl-4-methoxybenzophenone, 2-isopropyl Thioxanthone-based initiators including thioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2,4-dichlorothioxanthone, etc.; aminobenzophenone-based initiators including Michler's ketone, 4,4'-diethylaminobenzophenone, etc.; Included are 10-butyl-2-chloroacridone, 2-ethyl anthraquinone, 9,10-phenanthrenequinone, and camphorquinone.

Examples of cationic polymerization initiators include photoacid generators. Examples of photoacid generators include the aromatic onium compounds B(C ₆ F ₅ ) ₄ ⁻ _, PF ₆ ⁻ , AsF 6 − , SbF ₆ ^{− ,} CF, including diazonium, ammonium, iodonium, sulfonium, and phosphonium. Included are sulfonates that generate sulfonic acid, such as _{3SO 3} ^-salts , halides that photogenerate hydrogen halide, and iron arene complexes.

The content of the actinic ray polymerization initiator can be arbitrarily set within a range where the white ink is sufficiently cured by irradiation with actinic rays and the ejection properties of the white ink are not significantly reduced. For example, the white ink preferably contains an actinic radiation polymerization initiator of 0.1% by mass or more and 10% by mass or less, and 0.1% by mass or more and 8% by mass or less of an actinic radiation polymerization initiator based on the total mass of the white ink. It is more preferable to contain an initiator.

(wax)
The wax is a compound that does not have an actinic radiation polymerizable functional group and causes the actinic radiation curable ink to undergo a sol-gel phase change due to temperature changes. The wax turns the white ink into a sol when heated, and into a gel around room temperature. As a result, the heated and solized white ink is ejected from the inkjet head, and the white ink that is cooled and gelled upon landing on the recording medium is temporarily solidified, thereby improving the pinning property of the white ink. I can do it.

The wax is dissolved in the actinic radiation polymerizable compound contained in the white ink at a temperature higher than the gelling temperature of the white ink, and is dissolved in the white ink at a temperature lower than the gelling temperature of the white ink. Preferably, it is a compound that crystallizes. Gelation temperature is the temperature at which, when the white ink that has been turned into a sol or liquid by heating is cooled, the actinic radiation-curable ink undergoes a phase transition from sol to gel, and the viscosity of the white ink suddenly changes. means. Specifically, each time the white ink, which has been solized or liquefied, is cooled while measuring its viscosity with a rheometer (for example, MCR300 manufactured by Anton Paar), the temperature at which the viscosity suddenly increases is measured. The gelation temperature can be set to .

When the wax is crystallized with the above white ink, a structure may be formed in which the actinic radiation polymerizable compound is contained in the three-dimensional space formed by the wax crystallized in a plate shape (such a structure is referred to as below). (referred to as "card house structure"). When the card house structure is formed, the liquid actinic radiation polymerizable compound is held in the space, so that the dots formed by the white ink adhering to the recording medium become more difficult to wet and spread. Pinability is further improved. When the pinning property of the ink increases, it becomes difficult for dots formed by the ink to adhere to the recording medium to coalesce. Note that the above-described card house structure and the above-described orientation in the plane direction are compatible.

From the viewpoint of facilitating the formation of a card house structure, it is preferable that the actinic radiation-polymerizable compound and the wax are compatible with each other in the actinic radiation-curable ink.

In this embodiment, the white ink contains at least two types of wax, at least one of which is a wax having a branched structure. From the viewpoint of making the crystal size of the wax smaller and making the orientation of the crystals more random, thereby making it more difficult for cracks to occur, when the wax having the branched structure is a compound having an ester structure, the ester value is It is preferably a compound of 2 or more and 6 or less, more preferably a compound of 3 or more and 6 or less, and even more preferably a compound of 4 or more and 6 or less.

The ester value can be determined by the neutralization titration method described in JIS K0070 (1992).

Alternatively, the molecular structure can be estimated using infrared spectroscopy (FFIR method) and the ester value can be calculated from the estimated molecular structure.

The wax having a branched structure is at least selected from the group consisting of a compound having a pentaerythritol structure and an ester value of 2 or more, and a compound having a dipentaerythritol structure and an ester value of 2 or more. It contains one kind of compound, and may contain two or more kinds of compounds among these. The polyvalent ester compound included in the white color is preferably a compound having a branched structure and an ester value of 2 or more.

Compounds having a pentaerythritol structure and an ester value of 2 or more and compounds having a dipentaerythritol structure and an ester value of 2 or more are compounds represented by the following general formulas (1) to (11). However, it is not limited to the compounds represented by these general formulas (1) to (11) as long as the objects of the present invention are achieved and the effects of the present invention are achieved.

R ¹ to R ³⁹ in general formulas (1) to (11) each independently represent a linear alkyl group having 9 or more carbon atoms, and represent a linear alkyl group having 14 or more carbon atoms. is preferable, and more preferably represents a linear alkyl group having 16 or more carbon atoms. The upper limit of the number of carbon atoms in the alkyl group is not particularly limited, but is preferably 30 or less, more preferably 24 or less.

Examples of compounds having the above-mentioned pentaerythritol structure and having an ester value of 2 or more include pentaerythritol trimyristate (C13, general formula 3), pentaerythritol distearate (C17, general formula 2), pentaerythritol trimyristate (C13, general formula 2), These include erythritol tetrapalmitate ester (C15, general formula 3), pentaerythritol tetraarachidate ester (C19, general formula 1), and the like. Furthermore, the present invention is not limited to this, and esterification may be performed by mixing a plurality of types of fatty acids. Note that the number of carbon atoms in parentheses represents the number of carbon atoms in an alkyl group connected by an ester group.

Examples of compounds having the above dipentaerythritol structure and an ester value of 2 or more include dipentaerythritol tetralignoceric acid ester (C23, general formula 6 or 7), dipentaerythritol tetrabehenic acid ester (C21, general formula 6 or 7), dipentaerythritol hexastearate ester (C17, general formula 4), dipentaerythritol pentapalmitic acid ester (C15, general formula 5), dipentaerythritol tribehenic acid ester (C21, general formula 8 or 9), and dipentaerythritol dimyristic acid ester (C13, general formula 10 or 11). In addition, it is not limited to this, and multiple types of fatty acids may be mixed and esterified. The number of carbon atoms in parentheses indicates the number of carbon atoms of the alkyl group linked by the ester group.

In addition, in the compounds exemplified above, the number of carbon atoms in the alkyl groups connected by the ester group is the same, but the number of carbon atoms may be different among the plurality of alkyl groups.

Further, the wax having a branched structure is not limited to a compound having an ester group, and may have other structures such as a keto group, an amide bond, and a urethane bond.

The content of the wax having the branched structure may be any amount as long as the object of the present invention is achieved and the effect of the present invention is exhibited, but it is preferably 0.10% by mass or more and 1.0% by mass or less, more preferably 0.10% by mass or more and 0.90% by mass or less, more preferably 0.40% by mass or more and 0.80% by mass or less, more preferably 0.50% by mass or more and 0.70% by mass or less, and even more preferably 0.50% by mass or more and 0.60% by mass or less, based on the total mass of the white ink. By making the content of the wax having the branched structure 0.10% by mass or more, the crystal size of the wax can be made smaller and the crystal orientation can be made more random, making it less likely for cracks to occur. In addition, by making the content of the polyvalent ester compound 0.10% by mass or more, the crystal size can be made smaller, making the crystal arrangement more dense, and making it less likely for oxygen to penetrate the gaps between the crystals. Therefore, especially when a radical polymerizable compound is used as the actinic radiation polymerizable compound, oxygen inhibition can be suppressed and the curability of the white ink can be further improved. By setting the content of the polyvalent ester compound to 1.0% by mass or less, it is possible to suppress the growth of large crystals that would otherwise grow from wax with a branched structure, making it more difficult for cracks to occur.

Preferably, the white ink contains at least two types of waxes having branched structures, each having a different structure. This makes it easier to grow wax crystals in more directions, and makes it easier to disperse stress due to volumetric contraction that occurs when the ink lands in multiple directions, making it more difficult for cracks to occur.

Preferably, the white ink contains the wax that does not have the branched structure. This makes it easier to grow wax crystals in more directions, and makes it easier to disperse stress due to volumetric contraction that occurs when the ink lands in multiple directions, making it more difficult for cracks to occur.

Examples of waxes without a branched structure include paraffin wax, microcrystalline wax, petroleum wax such as petrolactam, candelilla wax, carnauba wax, rice wax, wood wax, jojoba oil, jojoba solid wax, and Vegetable waxes such as jojoba ester, animal waxes such as beeswax, lanolin and spermaceti, mineral waxes such as montan wax and hydrogenated wax, hydrogenated castor oil or hydrogenated castor oil derivatives, montan wax derivatives, paraffin wax derivatives, Modified waxes such as microcrystalline wax derivatives or polyethylene wax derivatives, higher fatty acids such as behenic acid, arachidic acid, stearic acid, palmitic acid, myristic acid, lauric acid, oleic acid, and erucic acid, and higher alcohols such as stearyl alcohol and behenyl alcohol. , hydroxystearic acid such as 12-hydroxystearic acid, 12-hydroxystearic acid derivatives; lauric acid amide, stearic acid amide, behenic acid amide, oleic acid amide, erucic acid amide, ricinoleic acid amide, 12-hydroxystearic acid amide, etc. fatty acid amide, N-substituted fatty acid amide such as N-stearyl stearic acid amide, N-oleyl palmitic acid amide, N,N'-ethylenebisstearylamide, N,N'-ethylenebis-12-hydroxystearylamide, and Special fatty acid amides such as N,N'-xylylene bisstearylamide; higher amines such as dodecylamine, tetradecylamine or octadecylamine, sucrose fatty acid esters such as sucrose stearic acid and sucrose palmitic acid, polyethylene wax, α - Includes synthetic waxes such as olefin maleic anhydride copolymer waxes, polymerizable waxes, dimer acids, dimer diols, etc. These waxes may be used alone or in combination of two or more.

Of these, the wax is preferably a higher fatty acid, a higher alcohol, a fatty acid ester, a fatty acid amine, an aliphatic ketone, or a fatty acid amide, more preferably a fatty acid ester or an aliphatic ketone, and even more preferably a compound represented by the following general formula (G1) or a compound represented by the general formula (G2).

General formula (G1): R ⁴⁰ -CO-R ⁴¹
General formula (G2): R ⁴² -COO-R ⁴³
In general formula (G1), R ⁴⁰ and R ⁴¹ independently represent an optionally branched linear hydrocarbon group having 9 to 25 carbon atoms, and in general formula (G2), R ⁴² and R ⁴³ independently represent a linear hydrocarbon group having 9 to 25 carbon atoms and which may have a branched chain.

Examples of the compound represented by the general formula (G1) include dilignoseryl ketone (carbon number: 23-24), dibehenyl ketone (carbon number: 21-22), distearyl ketone (carbon number: 17-18 ), dieicosyl ketone (carbon number: 19-20), dipalmityl ketone (carbon number: 15-16), dimyristyl ketone (carbon number: 13-14), dilauryl ketone (carbon number: 11-12) ), lauryl myristyl ketone (carbon number: 11-14), lauryl palmityl ketone (11-16), myristyl palmityl ketone (13-16), myristyl stearyl ketone (13-18), myristyl behenyl ketone (13-22) ), palmityl stearyl ketone (15-18), valmityl behenyl ketone (15-22) and stearyl behenyl ketone (17-22). Note that the number of carbon atoms in parentheses above represents the number of carbon atoms in each of the two hydrocarbon groups separated by the carbonyl group.

Examples of commercial products of the compound represented by general formula (G1) include 18-Pentatoriacontanon and Hentriacontan-16-on (both manufactured by Alfa Aeser), and Kaoh Wax T1 (manufactured by Kao Corporation). It will be done.

Examples of compounds represented by general formula (G2) include behenyl behenate (carbon number: 21-22), icosyl icosanoate (carbon number: 19-20), stearyl stearate (carbon number: 17-18) , palmityl stearate (carbon number: 17-16), lauryl stearate (carbon number: 17-12), cetyl palmitate (carbon number: 15-16), stearyl palmitate (carbon number: 15-18), myristin myristyl acid (carbon number: 13-14), cetyl myristate (carbon number: 13-16), octyldodecyl myristate (carbon number: 13-20), stearyl oleate (carbon number: 17-18), erucic acid These include stearyl (carbon number: 21-18), stearyl linoleate (carbon number: 17-18), behenyl oleate (carbon number: 18-22), and arachidyl linoleate (carbon number: 17-20). Note that the number of carbon atoms in parentheses above represents the number of carbon atoms of each of the two hydrocarbon groups separated by the ester group.

Examples of commercial products of the compound represented by the general formula (G2) include Unistar M-2222SL, Spam Acetyl, Nissan Erector WEP-2, and Nissan Erector WEP-3 (all manufactured by NOF Corporation, "Unistar") and "Nissan Elector" are both registered trademarks of the company), EXEPEARL SS and EXEPEARL MY-M (both manufactured by Kao Corporation, "EXEPEARL" is a registered trademark of the company), EMALEX CC-18 and EMALEX CC-10 ( These include Nippon Emulsion Co., Ltd., "EMALEX" is a registered trademark of the company), and Amreps PC (manufactured by Kyukyu Alcohol Kogyo Co., Ltd., "Amreps" is a registered trademark of the company). Since these commercially available products are often a mixture of two or more types, they may be separated and purified and included in the white ink if necessary.

The wax content is 0.1% by mass or more and 4.0% by mass or less based on the total mass of the white ink. By setting the wax content to 0.1% by mass or more, the pinning properties of the white ink can be sufficiently enhanced, and the conveyance properties of the recording medium due to wax precipitation can also be sufficiently enhanced. By controlling the wax content to 4.0% by mass or less, the blooming resistance of the ink can be improved, and the amount of wax near the surface of the cured film can be more appropriately controlled, thereby improving the curability of white ink. can be further increased. From the above viewpoint, the wax content is preferably 0.3% by mass or more and 3.2% by mass or less, and 0.5% by mass or more and 3.0% by mass or less based on the total mass of the white ink. It is more preferably 1.0% by mass or more and 2.9% by mass or less.

(Other ingredients)
The white ink may further contain other components such as a surfactant, a polymerization inhibitor, an actinic polymerization initiator, and a humectant.

(physical properties)
From the viewpoint of further improving ejection properties from an inkjet head, the viscosity of the white ink at 80° C. is preferably 3 mPa·s or more and 20 mPa·s or less.

It is preferable that the white ink has a phase transition temperature of 40° C. or more and 70° C. or less that causes a sol-gel phase transition. When the phase transition temperature of the white ink is 40° C. or higher, the white ink quickly thickens after landing on the recording medium, making it easier to adjust the degree of wetting and spreading. When the phase transition temperature of the white ink is 70°C or lower, the white ink is less likely to gel when ejected from the ejection head whose composition temperature is usually about 80°C, so the white ink is ejected more stably. be able to.

The viscosity and phase transition temperature at 80° C. of the white ink can be determined by measuring temperature changes in the dynamic viscoelasticity of the white ink using a rheometer.

(Method for preparing white ink)
The white ink can be prepared by mixing the above-mentioned components. At this time, in order to increase the solubility of each component, it is preferable to mix while heating.

Note that a pigment dispersion containing a white pigment, other optionally contained pigments, and a pigment dispersant may be prepared in advance, and the remaining components may be added to and mixed. Also at this time, in order to improve the solubility of the pigment dispersant, etc., it is preferable to prepare a pigment dispersion by mixing the pigment and the dispersant while heating.

[Image forming method]
An image forming method according to another embodiment of the present invention relates to a method of forming an image using the above-mentioned white ink. The image forming method described above can be carried out in the same manner as a conventionally known image forming method, except for using the above-mentioned white ink.

Specifically, the above image forming method includes 1) the step of attaching the above-mentioned white ink to a recording medium, and 2) irradiating actinic radiation to the white ink droplets attached to the recording medium to cause the droplets to and a step of curing.

(Process of attaching)
In the first step, white ink is applied to the recording medium at a position corresponding to the image to be formed.

The method of adhering to the recording medium is not particularly limited, and the white ink may be applied to the surface of the base material using a roll coater or spin coater, or may be spray coating, dipping, screen printing, gravure printing, offset printing. The white ink may be applied to the surface of the base material using a method such as the above, or the white ink may be made to land on the surface of the base material using an inkjet method. Among these, the inkjet method is preferred from the viewpoint of forming finer recorded matter.

The ejection method from the inkjet head may be either an on-demand method or a continuous method. On-demand inkjet heads include electro-mechanical conversion methods such as single cavity, double cavity, bender, piston, shear mode and shared wall, as well as thermal inkjet and bubble jet ("bubble jet"). '' is a registered trademark of Canon Inc.) type or other electric-to-thermal conversion method.

Further, the inkjet head may be either a scan type or a line type inkjet head, but a line type inkjet head is preferable.

The white ink droplets are heated and turned into a sol before being ejected from the inkjet head. From the perspective of improving the ejection performance of white ink from the inkjet head, the temperature of the white ink when filled into the inkjet head can be set to a temperature of gelling temperature of white ink +10°C or higher and gelling temperature of white ink +30°C or lower. preferable. When the temperature of the white ink in the inkjet head is at least 10° C. above the gelling temperature, the white ink is less likely to gel in the inkjet head or on the nozzle surface, thereby reducing the ejection performance. On the other hand, when the temperature of the white ink in the inkjet head is below the gelling temperature +30° C., deterioration of the components due to high temperatures is less likely to occur. Further, the viscosity of the white ink when ejected is preferably 7 mPa·s or more and 15 mPa·s or less, more preferably 8 mPa·s or more and 13 mPa·s or less.

The method of heating the white ink is not particularly limited. For example, at least one of the ink tanks constituting the head carriage, the ink supply system such as the supply pipe and the front chamber ink tank immediately in front of the head, the piping with a filter, and the piezo head is heated by a panel heater, ribbon heater, warm water, etc. be able to.

The amount of droplets of the white ink when ejected is preferably 2 pL or more and 20 pL or less from the viewpoint of further improving the recording speed and image quality.

Recording media are not particularly limited, and include ordinary uncoated paper, coated paper, synthetic paper Yupo ("Yupo" is a registered trademark of Yupo Corporation), various plastics used for flexible packaging, and their films. Can be used. Examples of various plastic films include PP film, PET film, OPS film, OPP film, ONy film, PVC film, PE film, and TAC film. Other plastics that can be used include polycarbonate, (meth)acrylic resin, ABS, polyacetal, PVA, and rubbers. Further, the recording medium may be metal, glass, or the like.

For adhering to the recording medium, the ejected white ink may be directly landed on the recording medium and attached, or the ejected white ink may be made to land on an intermediate transfer member to form an intermediate image, and the ejected white ink may be applied to the recording medium to form an intermediate image. The image may be transferred and attached to the recording medium from the intermediate transfer member.

(Curing process)
In the second step, the droplets of the white ink deposited on the recording medium in the first step are irradiated with actinic radiation to cure the droplets, thereby forming an image made of a cured film of the white ink.

The actinic radiation can be selected from, for example, electron beams, ultraviolet rays, alpha rays, gamma rays, and X-rays, but ultraviolet rays or electron beams are preferred. The ultraviolet rays preferably have a peak wavelength of 360 nm or more and 410 nm or less. Moreover, it is preferable that the ultraviolet rays are irradiated from an LED light source. LEDs emit less radiant heat than conventional light sources (such as metal halide lamps). Therefore, in the LED, when irradiated with actinic rays, the ink is hard to melt and uneven gloss is hard to occur.

[Image forming device]
An image forming apparatus according to yet another embodiment of the present invention is an image forming apparatus capable of implementing the above method.

FIG. 1 is a side view showing the concept of an image forming apparatus 100 according to this embodiment. Note that although the image forming apparatus 100 is an inkjet image forming apparatus, the present invention is not limited thereto.

As shown in FIG. 1, the image forming apparatus 100 includes an inkjet head 110, a transport section 120, and an irradiation section 130. Note that in FIG. 1, arrows indicate the conveyance direction of the recording medium.

(Inkjet head 110)
The inkjet head 110 has a nozzle surface 113 in which the ejection openings of the nozzles 111 are provided on a surface that faces the conveyance section 120 when forming an image, and has a nozzle surface 113 on a surface facing the conveyance section 120 when forming an image. to eject white ink. From the viewpoint of improving the ejection performance of the white ink, the inkjet head 110 may include a temperature adjustment means for adjusting the temperature of the ink and adjusting the ink to have a low viscosity. Examples of the temperature adjustment means include a panel heater, a ribbon heater, and a heating means using warm water.

The inkjet head 110 may be a scan type inkjet head whose width in the direction perpendicular to the conveyance direction of the recording medium is smaller than that of the recording medium 200, or a line whose width in the direction perpendicular to the conveyance direction of the recording medium is larger than the width of the recording medium 200. A type inkjet head may also be used.

The nozzle 111 has a discharge port on a nozzle surface 113. The number of nozzles 111 may be equal to or greater than the number of inks used for image formation (for example, four). When the inkjet head 110 has a plurality of nozzles 111, from the viewpoint of simplifying the configuration of the device and making control easier, the plurality of nozzles 111 are arranged in a line in the recording medium conveyance direction at approximately equal intervals. It is preferable that

The inkjet head 110 is configured to be able to change the amount of ink that is ejected and lands on the recording medium 200. For example, the inkjet head 110 is configured to be able to change the vibration width of the piezoelectric element or prevent ink from being ejected from some nozzles under the control of the control unit.

(Transport unit 120)
When forming an image, the conveyance unit 120 conveys the recording medium 200 so that the recording medium 200 facing the inkjet head 110 moves directly below the inkjet head 110 in the vertical direction. For example, the conveying unit 120 includes a driving roller 121, a driven roller 122, and a conveying belt 123.

The driving roller 121 and the driven roller 122 are arranged at a predetermined interval in the conveying direction of the recording medium 200 and extending in a direction perpendicular to the conveying direction of the recording medium 200. The drive roller 121 is rotated by a drive source (not shown).

The conveyance belt 123 is a belt for conveying the recording medium 200 placed thereon, and is spanned over the drive roller 121 and the driven roller 122. The conveyor belt 123 can be, for example, an endless belt that is wider than the recording medium 200. At this time, when the drive source rotates the drive roller 121, the conveyor belt 123 rotates following the drive roller 121, and the recording medium 200 on the conveyor belt 123 is conveyed.

(Irradiation section 130)
The irradiation unit 130 has a light source and irradiates the upper surface of the transport unit 120 with actinic rays from the light source. As a result, the white ink droplets that have landed on the conveyed recording medium 200 can be irradiated with actinic radiation to harden the droplets. The irradiation unit 130 can be disposed directly above the transport unit 120 on the downstream side of the inkjet head 110.

(Other configurations)
In addition to the above configuration, the image forming apparatus 100 also includes an ink tank (not shown) for storing white ink before ejection, and an ink flow path (not shown) that communicates the ink tank and the inkjet head 110 so that ink can flow. ), and a control unit (not shown) that controls the operations of the inkjet head 110, the transport unit 120, and the irradiation unit 130.

The image forming apparatus 100 may also have an intermediate transfer body and a transfer unit (neither shown). At this time, the inkjet head 110 ejects white ink onto the intermediate transfer body, causing it to land on the surface of the intermediate transfer body, and an intermediate image consisting of a collection of white ink droplets is formed on the surface of the intermediate transfer body. Thereafter, the transfer unit transfers the intermediate image from the surface of the intermediate transfer body to the surface of the recording medium. Then, the irradiation unit 130 irradiates the intermediate image transferred to the surface of the recording medium with active rays, thereby hardening the white ink droplets.

The present invention will be explained in more detail below with reference to Examples, but the scope of the present invention is not interpreted to be limited by these descriptions.

1. Preparation of actinic radiation curable inkjet ink 1-1. Materials An actinic radiation-curable inkjet ink was prepared using the following components (gelling agent (wax), actinic radiation polymerizable compound, polymerization inhibitor, actinic radiation polymerization initiator, pigment dispersion).

[Geling agent (wax)]
(straight chain wax)
Behenic acid (Lunac BA, NOF)
Behenyl behenate (Unistar M-2222SL NOF)
(branched wax)
Pentaerythritol trimyristate ester Pentaerythritol distearate ester (Unistar H-476D NOF)
Dipentaerythritol tetralignoseric acid ester

(Synthesis of pentaerythritol myristate ester)
A flask was charged with 136.2 parts by mass of pentaerythritol, 913.6 parts by mass of myristic acid, 19.6 parts by mass of concentrated sulfuric acid, and 2 L of toluene, and an esterification reaction was carried out at 80° C. for 4 hours. Next, separation by ester value was performed by column chromatography to obtain 38 parts by mass of pentaerythritol trimyristate.

(Synthesis of dipentaerythritol tetralignoceric acid ester)
A flask was charged with 254.3 parts by mass of dipentaerythritol, 2948.8 parts by mass of lignoceric acid, 19.6 parts by mass of concentrated sulfuric acid, and 2 L of toluene, and an esterification reaction was carried out at 80° C. for 4 hours. Next, separation according to ester value was performed by column chromatography to obtain 46 parts by mass of dipentaerythritol tetralignoceric acid ester.

[Active radiation polymerizable compound]
Polyethylene glycol diacrylate 4EO modified hexanediol diacrylate 6EO modified trimethylolpropane triacrylate

[Polymerization inhibitor]
Irgastab UV10 (manufactured by BASF)

[Active ray polymerization initiator]
IRGACURE 819 (manufactured by BASF)

[White pigment]
Titanium oxide without surface treatment (R-32 manufactured by Sakai Chemical Industry Co., Ltd.)

[Magenta pigment]
Pigment. Red 57:1 (manufactured by Dainichiseika Kagyo Co., Ltd.)

[Cyan pigment]
Pigment. Blue 15:3 (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.)

[Surface treatment agent]
Dimethylpolysiloxane Methyltrimethoxysilane Trimethylolpropane Zirconia

1-2. Pigment Surface Treatment A slurry prepared by adding 3.0 parts by mass of dimethylpolysiloxane as a surface treatment agent and 97.0 parts by mass of unsurface-treated titanium oxide into a solvent was mixed with a stirrer. The mixture was mixed and further subjected to a dispersion treatment for 5 minutes using a horizontal continuous sand grinder mill (manufactured by Amex Corporation, Ultra Visco Mill). The mixture after the above dispersion treatment was put into a kneader and heated under reduced pressure to remove the solvent. Thereafter, the obtained powder was cured at 120 to 150° C. to obtain surface-treated titanium oxide 1 in powder form.

A powdered surface-treated titanium oxide 2 was obtained in the same manner, except that the type of surface treatment agent was changed to methyltrimethoxysilane.

Powdered surface-treated titanium oxide 3 was obtained in the same manner except that the type of surface treatment agent was changed to trimethylolpropane.

Powdered surface-treated titanium oxide 4 was obtained in the same manner except that the type of surface treatment agent was changed to zirconia.

1-3. Preparation of Pigment Dispersion The following dispersant, photopolymerizable compound, and polymerization inhibitor were placed in a stainless steel beaker and heated and stirred for 1 hour on a hot plate at 65° C. to dissolve them. After the obtained solution was cooled to room temperature, 50% by mass of the following white pigment was added, and the mixture was placed in a glass bottle together with 200 g of zirconia beads having a diameter of 0.5 mm, tightly stoppered, and dispersed in a paint shaker for 8 hours. Thereafter, the zirconia beads were removed to prepare a white pigment dispersion, a cyan pigment dispersion, and a magenta pigment dispersion having the following compositions.

[White pigment dispersion]
Dispersant: Ajisper PB824 (manufactured by Ajinomoto Fine Techno) 10% by mass
Active radiation polymerizable compound: 3PO modified trimethylolpropane triacrylate 40% by mass
White pigment: 50% by mass of any one of surface-treated titanium oxide 1 to 4

[Magenta pigment dispersion]
Dispersant: Ajisper PB824 (manufactured by Ajinomoto Fine Techno) 10% by mass
Active radiation polymerizable compound: 3PO modified trimethylolpropane triacrylate 40% by mass
Magenta pigment: Pigment. Red 57:1 50% by mass

[Cyan pigment dispersion]
Dispersant: Ajisper PB824 (manufactured by Ajinomoto Fine Techno) 10% by mass
Active radiation polymerizable compound: 3PO modified trimethylolpropane triacrylate 40% by mass
Cyan pigment: Pigment. Blue 15:3 50% by mass

1-4. Preparation of Ink A mixture obtained by mixing each component according to the composition shown in Tables 1 to 5 below was heated to 80° C. and stirred. The obtained solution was filtered through a #3000 metal mesh filter under heating, and then cooled to prepare ink, and samples 1 to 16 were obtained. In Tables 1 to 5, the unit of blending amount of each component is mass %.

2. Image Formation A monochrome image was formed using any of Samples 1 to 16 using a line type inkjet recording device. The temperature of the inkjet head of the inkjet recording device was set at 80°C. A solid image of 5 cm x 5 cm was printed on a recording medium. After the image was formed, the ink was cured by irradiating the image with ultraviolet rays using an LED lamp (395 nm, water-cooled LED, manufactured by Phoseon Technology) placed downstream of the recording device. A piezo head with a nozzle diameter of 20 μm and 512 nozzles (256 nozzles x 2 rows, staggered arrangement, nozzle pitch per row of 360 dpi) was used as the ejection recording head. The ejection conditions were such that the amount of each droplet was 2.5 pl, ejection was made at a liquid speed of about 6 m/s, and recording was performed at a resolution of 1440 dpi x 1440 dpi. The recording speed was 500 mm/s. Image formation was performed in an environment of 23° C. and 55% RH. DPI represents the number of dots per inch (2.54 cm).

3. Image evaluation (evaluation of scales and cracks)
By the above method, cracks formed in a 5 cm x 5 cm white solid image formed on printing coated paper A (OK ^Top Coat, weight 128 g/m, manufactured by Oji Paper Co., Ltd.), which is a recording medium, were visually and microscopically observed, Cracks were evaluated according to the following criteria.
Rank 5: No cracks on the surface Rank 4: Fine cracks are observed in some parts when observed with a microscope, but are not visible to the naked eye Rank 3: Fine cracks are observed when observed with a microscope, but are not visible to the naked eye No Rank 2: Some small cracks are visible to the naked eye Rank 1: Visible to the entire surface

(Evaluation of storage stability)
The ink prepared by the above method was stored for two weeks in a constant temperature bath at 100° C., and the storage stability was evaluated for the viscosity of the ink after storage according to the following criteria.
○: Ink viscosity increase after storage is less than +1 cp ×: Ink viscosity increase after storage is +1 cp or more

(Surface hardenability evaluation)
By the above method, a force gauge (manufactured by Imada Co., Ltd.) was applied to a 5 cm x 5 cm white solid image formed on printing coated paper A (OK Top Coat, weight 128 g/m, manufactured by ² Oji Paper Co., Ltd.), which is a recording medium. The coefficient of kinetic friction was measured when the material was moved at a constant speed while applying an overload of 500 g, and the surface hardening property was evaluated according to the following criteria.
○: Dynamic friction coefficient is less than 0.1 ×: Dynamic friction coefficient is 0.1 or more

(Results and Discussion)
Images using the actinic radiation-curable inkjet inks of Samples 1 to 11 showed good storage stability. This is thought to be because the surface of titanium oxide is hydrophobically treated, which makes it easier for the dispersant to adsorb onto the pigment surface, thereby suppressing aggregation of the pigments.

Images using the actinic light-curable inkjet inks of Samples 1 to 11 showed good surface curability. Since this contains at least one branched wax among the plurality of waxes, the crystal size becomes small and the crystal arrangement becomes dense. This is thought to be because as a result, it became less susceptible to oxygen inhibition and polymerization was promoted.

In the images using the actinic light-curable inkjet inks of Samples 1 to 11, the generation of cracks was reduced more than in the images using the actinic light-curable inkjet inks of Samples 12 to 16. Images using the actinic radiation curable inkjet ink of Sample 1 were less prone to cracking than the actinic radiation curable inkjet inks of Samples 2 and 3, which had a larger amount of branched wax. Furthermore, images using the actinic radiation curable inkjet ink of Sample 1 were less prone to cracking than the actinic radiation curable inkjet inks of Samples 4 and 5, which had a smaller amount of branched wax.

In images using actinic radiation-curable inkjet inks of samples 6 to 7, cracking was improved to an equal or greater extent than in sample 1. This is believed to be because the inclusion of branched wax with two different structures makes it easier for stress to be dispersed in multiple directions. Sample 7 also had less cracking than sample 8, which contains the same type of branched wax as sample 7 but does not contain linear wax. This is believed to be because the inclusion of linear wax makes it easier for the wax crystals to grow in more directions, making it easier to disperse the stress caused by volumetric shrinkage that occurs when the ink lands in multiple directions.

The actinic radiation-curable inkjet inks of Samples 9 to 11 have the same composition as the actinic radiation-curable inkjet inks of Samples 2 to 4, respectively, except that dimethylpolysiloxane, which is the same as Sample 1, is used as the surface treatment species. It is. In the images using actinic radiation-curable inkjet inks of Samples 9 to 11, the cracking results showed the same results as those of samples 2 to 4 of actinic radiation-curable inkjet inks, so the difference due to the type of hydrophobizing surface treatment is It is considered to be extremely rare.

On the other hand, in the image using the actinic radiation curable inkjet ink of Sample 12, cracks were likely to occur. This is thought to be because the surface of the titanium oxide pigment was hydrophilized with zirconia, which created voids between the titanium oxide and the wax. On the other hand, the surface curability was high, probably because the pigment did not attract hydrophobic wax easily and the wax precipitated on the surface of the cured film.

In the actinic radiation-curable inkjet inks of Samples 13 to 14, cracking was reduced compared to Sample 12, probably because the surface of the titanium oxide was hydrophobized. However, since sample 13 does not contain branched wax, it is susceptible to oxygen inhibition. As a result, polymerization was not promoted and the surface hardening property was lower than that of Sample 12.

In addition, in the image of sample 14 using actinic radiation curable inkjet, only one type of wax was used, which is a branched wax. Therefore, crystals grow between the branched waxes, and although the orientation is random, the crystal size is thought to be large. As a result, it is more susceptible to the effects of oxygen inhibition, and the surface curability is lower than that of sample 12.

The actinic radiation-curable inkjet inks of Reference Examples 1 and 2 used magenta pigments and cyan pigments other than white, and the generation of cracks was suppressed regardless of the presence or absence of branched wax. This is thought to be because the pigment itself of inks of colors other than white is easily compatible with wax and suppresses the crystal growth of wax. When the present inventors observed the crystal size of wax, they confirmed that the crystal size of colored ink was smaller than that of white ink.

The actinic radiation-curable inkjet ink of the present invention is less likely to crack when a solid image is formed. Therefore, the present invention is expected to expand the scope of application of actinic radiation-curable inkjet inks to the formation of underlayers, especially the formation of solid white images, and is expected to contribute to the advancement and popularization of technology in this field. be done.

REFERENCE SIGNS LIST 100 Image forming apparatus 110 Inkjet head 111 Nozzle 113 Nozzle surface 120 Conveying section 121 Driving roller 122 Driven roller 123 Conveying belt 130 Irradiation section 200 Recording medium

Claims (8)

A white pigment treated with hydrophobization,
an actinic radiation polymerizable compound;
at least two types of wax,
An actinic radiation curable ink comprising:
At least one of the at least two types of waxes is a wax having a pentaerythritol structure or a dipentaerythritol structure ,
The wax having a pentaerythritol structure or dipentaerythritol structure has an ester value of 2 or more,
The wax causes a sol-gel phase transition due to temperature changes.
Actinic radiation curable ink. The actinic radiation-curable ink according to claim 1, wherein the white pigment is surface-treated with one selected from the group consisting of a polyol, a siloxane compound, and a silane coupling agent. According to claim 1 or 2 , the content of the wax having a pentaerythritol structure or dipentaerythritol structure is 0.10% by mass or more and 1.00% by mass or less based on the total mass of the actinic radiation curable ink. The actinic radiation curable ink described above. 4. The actinic ray curable ink according to claim 1, wherein the wax having a pentaerythritol structure or a dipentaerythritol structure includes at least two kinds of wax having mutually different structures. The actinic radiation curable ink according to any one of claims 1 to 4 , wherein the wax includes a wax that does not have a pentaerythritol structure or a dipentaerythritol structure . The actinic radiation curable ink according to any one of claims 1 to 5, which is an inkjet ink. A step of applying the actinic radiation curable ink according to any one of claims 1 to 6 to a recording medium,
irradiating the applied actinic radiation curable ink with actinic radiation;
An image forming method, including: The image forming method according to claim 7 , wherein the applying step is performed by an inkjet method.

Images